Various analytical methods have been known for quantitatively determining a specific constituent in a liquid by detecting hydrogen peroxide resulting from a reaction involving the specific constituent. Among them are methods of using leuco dyes containing an imidazole nucleus as described in U.S. Pat. No. 4,089,747 and Japanese Patent Application (OPI) No. 193352/1984 ("OPI" means a published unexamined Japanese patent application).
These leuco dyes produce 1 mole of dye from 1 mole of hydrogen peroxide, and since the dye has a high molecular extinction coefficient, the detection sensitivity is advantageously high. However, it is disadvantageous that the discoloring of the dye is relatively high.